This invention relates generally to the field of motion systems and more specifically to a low-cost, high precision goniometric stage for use in x-ray diffracto-graphy or optical systems. Goniometers have been used for many years to measure angular relationships begriming with Wollaston's goniometer (1807) for measuring the angle between crystal faces. Recently single-tilt and double-tilt sample holders have been widely used in electron microscopy and in x-ray diffractometers. In most applications the goniometer rotates a sample around two axes and is often coupled with rotational and translational stages to provide up to six degrees of freedom.
As x-ray diffractography has advanced the need for precise sample positioning has become more stringent. Analyzer crystals, zone plates, polarization analyzers, quarter-wave plates and small mirrors must be rotated to micro- and nano-radian accuracy. Stability and repeatability have also become increasingly important as analysis times can require several hours or more.
Various techniques are currently used to solve this positioning problem. Most schemes rely upon specialty mechanisms such as stacks of circles, arc-segments or goniometric cradles machined to different radii that produce axes with orthogonal rotations about a point. Such devices are commercially available from, for example, Huber Diffraktionstechnik GmbH. There are usually substantial stages and fixtures mounted on these positioners to accommodate large crystals, water cooling mounts or fast response piezo-driven stages. Other approaches to positioning have been proposed but not widely adopted.
Goldowsky, U.S. Pat. No. 4,759,130 (1988) developed a goniometer head with a resolution of 0.001°. Displacements were applied to a cantilevered rod in such a way that its free end was not displaced but could be rotated about two orthogonal axes. Neither the resolution nor the size of the stage are, however, satisfactory.
Gimbal mounts allow two axes of rotation and have been used in many applications. Gimbals mounts were used by Alani et al, U.S. Pat. No. 6,388,262 (2002) in an application for transmission electron microscopes. No resolution was stated, however, gimbal mounts with a resolution of less than 1 arc-sec are widely available.
“Optical mount with independently orthogonally adjustable element”, U.S. Pat. No. 4,088,396—Edelstein (1978) teaches the use of an optical mount which employs a spherical bearing surface. This is properly termed a kinematic device since the axes of rotation are located behind the optical component. Several subsequent patents use the spherical bearing but move the rotational axes to intersect on the optical component's surface. These include: Romero in “Gimbal Assembly”, (U.S. Pat. No. 4,938,564—1990); Pong in “Adjusting Mechanism for a lens”, (U.S. Pat. No. 5,138,496—1992)and Kimura et al in “Lens frame supporting mechanism” (U.S. Pat. No. 5,502,598—1996). They differ in the method of adjustment. A patent by Dallakian, U.S. Pat. No. 6,198,580 (2001), also uses a spherical bearing but achieves superior resolution with his adjusting means by providing a means to bias the bearing. A commercially available version of this design, available from Newport Corp., has a resolution of 0.3 arc sec when differential micrometers are used for adjustment.
A goniometer stage for use in x-ray diffractography is taught in U.S. Pat. No. 5,475,728 “Eucentric motion system”, Smith et al (1995). It consists of a triangular plate, with a sample stage at the center, supported on three vertical actuators. In one embodiment a spherical bearing is mounted underneath the stage and biased with springs. The spherical bearing prevents in-plane movement of the stage, however, it is located on a sliding shaft that allows vertical movement making this a kinematic type mount. As a consequence all three actuators must be controlled in order to maintain a constant sample height. Significantly, the spherical bearing is designed to have its center of rotation above the stage.
Existing designs for goniometers are unnecessarily large, complex and costly. All of the devices described employ means of adjustment that project beyond the area that may be used for sample mounting. Excepting Smith et al all of the devices that utilize a spherical bearing surface rely on direct surface to surface contact which, due to static frictional forces, reduces their precision.
Gimbal type mounts and those optical mounts with spherical bearings are not suitable since the axes of rotation are below the sample. This necessitates simultaneous in-plane and vertical adjustments when used for x-ray diffractography and introduces additional positioning error.
The eucentric motion system of Smith et al correctly locates the axes of rotation above the sample stage and utilizes a spherical bearing surface supported on ball-bearings to constrain certain motions. It is still, however, unnecessarily large and complex and requires vertical correction.